Wells are generally drilled into subsurface rocks to access fluids, such as hydrocarbons, stored in subterranean formations. The formations penetrated by a well can be evaluated for various purposes, including for identifying hydrocarbon reservoirs within the formations. During drilling operations, one or more drilling tools in a drill string may be used to test or sample the formations. Following removal of the drill string, a wireline tool may also be run into the well to test or sample the formations. These drilling tools and wireline tools, as well as other wellbore tools conveyed on coiled tubing, drill pipe, casing or other means of conveyance, are also referred to herein as “downhole tools.” Certain downhole tools may include two or more integrated collar assemblies, each for performing a separate function, and a downhole tool may be employed alone or in combination with other downhole tools in a downhole tool string.
Formation evaluation can involve drawing fluid from the formation into a downhole tool. A pump in the downhole tool can be used to initiate a drawdown to cause fluid to enter the downhole tool from the formation, to route this formation fluid within the tool, and to expel the fluid into the wellbore. Once drawn from the formation, the fluid can be analyzed within the tool or samples of the fluid can be stored within the tool for later analysis. In some instances, downhole tools include pumps having reciprocating pistons for drawing formation fluid into the tool and check valves are used to maintain flow of the formation fluid in a desired direction through the tool. During normal pump operation, the volume of fluid drawn from the formation can be generally calculated based on the volume of fluid displaced by the piston. Malfunctioning of one or more check valves in the tool, however, can result in half-stroking of the pump. During half-stroking, the ability of the pump to draw fluid from the formation is impaired and the volume of fluid actually drawn from the formation is less than the volume of fluid displaced by the piston.